Electric Power Steering (EPS) systems use an electric motor that can be coupled directly to either the steering gear or steering column to reduce a driver's effort in steering the vehicle. To explain further, during operation of the vehicle, the driver will apply a force to the steering wheel (SW) in an effort to steer the vehicle. This results in a “driver torque” being applied to a shaft that is coupled to the SW. Torque sensors detect torque being applied to the steering column by the driver, and communicate this information to an electronic control unit. The electronic control unit generates a motor control signal that is applied to the electric motor causing it to generate a “motor assist torque” that is combined with the driver torque. This combined torque is then used to steer the “corners” of the vehicle. This allows varying amounts of assistance to be applied depending on driving conditions.
There are numerous types of unwanted vibrations, noises, pulsations, disturbances, and other forms of fluctuating vibratory energy that can exist in a vehicle; these phenomena are hereafter collectively and broadly referred to as “vibrations.” Vibrations can have many sources, including external sources such as irregular road surfaces, as well as internal sources.
When a vehicle operates at typical highway speeds (e.g., 72.42 to 144.84 kilometers per hour, or 45 to 90 miles per hour), the excitation of irregularities at the corners of a vehicle can result in internally-generated periodic torsional vibrations at the vehicle's steering wheel. As used herein, the term “corner” refers to parts at the roadwheel positions of a vehicle from the tie-rod outward. The parts that make up a corner may include a tire, wheel, brake rotor, hub bearing assembly, control arm, knuckle, bushings, etc. An example of an internal source of vibrations is a non-concentric, out-of-round, or otherwise irregular rotating part. For instance, if a tire, wheel, hub and/or rotor is manufactured or mounted to the vehicle in a non-concentric or off-balance manner, then the part rotates with an uneven weight distribution. This, in turn, can produce periodic or harmonic vibrations in the vehicle; that is, vibrations having a first order component centered at a first order frequency, as well as higher- or multi-order components centered at frequencies that are integer multiples of the first order frequency. A first order component of a periodic vibration is centered at the same frequency as the rotating object from which it emanates and, for steering systems, usually has a greater amplitude or intensity than its higher-order counterparts. For example, a wheel rotating at fifteen rotations per second (15 Hz) can produce periodic vibrations having a first order component at 15 Hz, a second order component at 30 Hz, a third order component at 45 Hz, and so on. The first order or 15 Hz component is usually more intense than the second and third order components. It should be appreciated that non-concentric rotating parts are only one potential source of periodic vibrations in a vehicle, as many others also exist.
Periodic vibrations caused by internal sources can propagate throughout the vehicle and can cause an undesirable shake or movement of certain vehicle components that is noticeable to the driver. For instance, periodic vibrations generated at the wheel assemblies or corners can combine to create a dynamic torque on a steering wheel column component that causes the steering wheel to cyclically turn at small amplitudes in either direction. When this type of event occurs on a flat or smooth road surface, it is all the more apparent to the driver and is sometimes referred to as “smooth road shake (SRS)” or “torsional nibble”. These vibrations can be sensed by the driver of the vehicle and will be referred to herein as steering wheel vibrations (SWVs). The frequencies of SWVs are usually proportional to speed and a first order harmonic of the tire rotation frequency (e.g., the rolling frequency of the tire). Dynamic amplitudes are small, near or in excess of the thresholds of perception at 0.03 degrees.
A variety of techniques for reducing or mitigating periodic vibrations in the vehicle have been developed. These techniques include on- and off-vehicle wheel balancing, using different types of damping components, and attempting to machine or otherwise produce more concentric and precise parts. In vehicles that implement electronic power steering systems (EPSs), EPS-control algorithms have been developed that allow for active-SWV suppression methods to be implemented, but these algorithms may be more costly, and require new hardware and/or wiring, or they may not be as effective for suppression. These algorithms may also interfere with normal driving operation (i.e., effect the perception of the steering performance as sensed by the driver), and/or render the system less stable (in a vibratory sense) by producing larger aperiodic SWVs on rough roads, or achieving vibratory limit-cycle conditions.
Accordingly, it would be desirable to provide improved methods, systems and apparatus for suppressing SWVs in an EPS system. It would be desirable if such methods, systems and apparatus do not require additional hardware or wiring such that they can be applied generally in many types of vehicles without requiring substantial modifications. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.